In recent years, there is a strong desire to improve automotive crash safety, and, in particular, installation of safety devices for protecting occupants in a collision is actively promoted. Specifically, airbags are becoming more common. At the time of a collision, an airbag inflates between an occupant and the steering wheel or the instrument panel, and the kinetic energy of the occupant is thereby absorbed to reduce the damage to the occupant. In particular, a driver-side airbag housed in the steering wheel and a passenger-side airbag housed in the instrument panel are becoming standard equipment. Recently, in addition to these airbags, side airbags installed in seats and curtain airbags configured to cover side windows are mounted on many automobiles to protect occupants at the time of a side collision. An inflator for an airbag (hereinafter referred to as an airbag inflator) includes a bottle serving as a container.
Methods used for many conventional airbags include a method in which an explosive contained in an inflator is used to generate gas and a method in which the thermal expansion of reaction gas contained in an inflator is used. In one recently used method, an inflator is filled with an inert gas such as argon at high pressure instead of using an explosive, in consideration of recycling and environment.
Generally, a bottle for an airbag is manufactured by processing a steel pipe. The steel pipe is subjected to cold drawing to obtain prescribed dimensions, and the resulting steel pipe is cut to a prescribed length. Then both ends of the pipe are subjected to, for example, press working, and seal plates are welded to thereby obtain a bottle. Therefore, there is a need for a steel pipe for airbag inflators that has sufficient strength and toughness and is excellent in workability and weldability.
In view of the above need, for example, Patent Literature 1 describes a method for manufacturing a high-strength, high-toughness steel pipe for airbags. The method includes: preparing a pipe using steel containing C: 0.01 to 0.20%, Si: 0.50% or less, Mn: 0.30 to 2.00%, P: 0.020% or less, S: 0.020% or less, and Al: 0.10% or less and further containing at least one of Mo: 0.50% or less, V: 0.10% or less, Ni: 0.50% or less, Cr: 1.00% or less, Cu: 0.50% or less, Ti: 0.10% or less, Nb: 0.10% or less, and B: 0.005% or less, with the balance being Fe and incidental impurities; then subjecting the pipe to cold working; and optionally subjecting the cold-worked pipe to annealing, normalizing, or quenching and tempering. The steel pipe described in Patent Literature 1 may be an electric resistance welded steel pipe or may be a seamless steel pipe. The steel pipe obtained has high dimensional accuracy, is excellent in workability and weldability, has high strength and high toughness, and can be used for accumulators of airbags (bottles of airbags).
Patent Literature 2 describes a method for manufacturing a high-strength, high-toughness steel pipe for airbags. The method includes: preparing a pipe using steel containing C: 0.05 to less than 0.15%, Si: 0.50% or less, Mn: 0.30 to 2.00%, P: 0.020% or less, S: 0.020% or less, and Al: 0.1% or less and further containing at least one of Mo: 0.50% or less, V: 0.10% or less, Ni: 0.50% or less, Cr: 1.00% or less, Cu: 0.50% or less, Ti: 0.10% or less, Nb: 0.10% or less, and B: 0.005% or less, with the balance being Fe and incidental impurities; normalizing the pipe at 850 to 1,000° C.; cold-working the resulting pipe to prescribed dimensions; and optionally subjecting the cold-worked pipe to stress relief annealing, normalizing, or quenching and tempering. The steel pipe described in Patent Literature 2 may be an electric resistance welded steel pipe or may be a seamless steel pipe. The steel pipe obtained has high dimensional accuracy, is excellent in workability and weldability, has high strength and high toughness, and can be used for accumulators of airbags etc.
Patent Literature 3 describes a method for manufacturing a high-strength, high-toughness steel pipe for airbags. The method includes: preparing a pipe using steel containing C: 0.05 to less than 0.15%, Si: 0.50% or less, Mn: 0.30 to 2.00%, P: 0.020% or less, S: 0.020% or less, and Al: 0.1% or less and further containing at least one of Mo: 0.50% or less, V: 0.10% or less, Ni: 0.50% or less, Cr: 1.00% or less, Cu: 0.50% or less, Ti: 0.10% or less, Nb: 0.10% or less, and B: 0.005% or less, with the balance being Fe and incidental impurities; quenching the pipe from 850 to 1,000° C.; tempering the resulting pipe at a temperature of 450° C. or higher and lower than Ac1 transformation temperature; cold-working the tempered pipe to prescribed dimensions; and optionally subjecting the cold-worked pipe to annealing. The steel pipe described in Patent Literature 3 may be an electric resistance welded steel pipe or may be a seamless steel pipe. The steel pipe obtained has high dimensional accuracy, is excellent in workability and weldability, has high strength and high toughness, and can be used for accumulators of airbags etc.